1. Field of the Invention
The present invention generally relates to a sampling apparatus which samples analog signals and digitizes and outputs them; in particular, it concerns a sampling apparatus with a plurality of input ranges.
2. Discussion of the Background Art
Analog/digital converters (referred to below as xe2x80x9cADCxe2x80x9d) are often used in the signal receiving parts of electronic measuring apparatuses. Since sampled signals can have high dynamic ranges, it is beneficial that the whole amplitude of the signal input to the ADC is close to the full scale input value of the ADC. Sampling apparatuses may be constructed in electronic measuring apparatuses by placing an amplifier before the ADC. In this instance, the whole amplitude of the signal input to the ADC is amplified so that it is close to the full scale input value of the ADC.
FIG. 1 shows a conventional sampling apparatus which is provided with an amplifier and has a plurality of input ranges. In FIG. 1, the sampling apparatus 100 is provided with an input terminal 110, a switch 120, an amplifier 130, and ADC 140, and an output terminal 150.
The amplifier 130 is placed between the input terminal 110 and the switch 120 and amplifies the amplitude of the signal input from the input terminal 110, and outputs it to the switch 120.
The switch 120 connects the input terminal 110, the amplifier 130, and the ADC 140; it selectively connects either the input terminal 110 or the amplifier 130 to the ADC 140.
The ADC 140 samples the input signal and converts the analog signal values to digital data, which are output to output terminal 150.
The sampling apparatus 100, which is constructed as described above has 2 input ranges. That is, the signal that is input into the input terminal 110 is either sampled directly or after being amplified. Here, the setting at which the signal input to the input terminal 110 is directly sampled is called the xe2x80x9clow-gain range,xe2x80x9d and the setting at which this input signal is amplified before sampling is called the xe2x80x9chigh-gain range.xe2x80x9d
When a signal with a large amplitude is sampled in the high-gain range in such a sampling apparatus 100 with 2 input ranges, the instantaneous values of the signal sometimes exceed the input range of the ADC 140 and values cannot be sampled effectively. Therefore, in a sampling apparatus 100 which has a plurality of input ranges, an operation of switching to a suitable range for sampling is necessary. This operation is generally called xe2x80x9cranging.xe2x80x9d
An example of a means generally used in ranging with the sampling apparatus 100 shown in FIG. 1 is as follows. First, the range is switched to the low-gain range. Then, sampling is performed by the ADC 140. The sampling is performed for 1 cycle of the signal input to the sampling apparatus 100. If the input signal is a symmetrical wave, such as a sine wave signal, half of a cycle is enough. Next, a value which exceeds the input range of the ADC 140 when the sampling is performed in the high-gain range is taken as a threshold value; all of the sample values are compared with this threshold value, and if all of the sample values are below the threshold value, the switch is thrown to the high-gain range and regular sampling is performed. If at least 1 sample value is at or above the threshold value, regular sampling is performed with the setting still at the low-gain range.
Thus, in conventional ranging means, it is necessary to input at least one-half of a period into the sampling apparatus to perform the sampling for ranging. This time becomes longer the longer the period of the input signal is. In some cases switching is performed, so that time is also required for the over-responses of the signal caused by the switching to converge. These times become part of the measurement time of the electronic measuring apparatus, and are factors which limit the degree to which the measurements can be accelerated.
Since measurement times are factors which are reflected in manufacturing costs, etc., it is desirable to shorten measurement times, and it is desirable to shorten the times required for ranging and sampling in sampling apparatuses.
The present invention uniquely eliminates the sampling operation performed for ranging and shortens the time required for ranging by sampling signals simultaneously by a number of sampling means in a sampling apparatus provided with two (2) or more sampling means and uses the values sampled by one of these sampling means. Moreover, the present invention unexpectedly infers the possibility that an instantaneous value of a signal input into a sampling means will exceed the input range of the sampling means by establishing a suitable threshold for the values of the samples obtained by this sampling means.
A sampling apparatus comprising at least two (2) sampling means and a selection means for selecting the sampling means which uses the sampled values of the sampling means selected by the selection means as its output. The sampling means sample the same sine wave signal which has a known frequency at N (3 or more) points per cycle. The selection means selects the sampling means with the narrowest input range among the sampling means such that the absolute values of the samples of the sine wave signal are all below a threshold value. The threshold value is sin (xcfx80/2xe2x88x92xcfx80/N) times one-half the input range of the sampling means when N is even, and sin [xcfx80/2xe2x88x92xcfx80/(2N)] times one-half the input range of the sampling means when N is odd.
An apparatus is provided herein with a sampling means and a comparison means which makes inferences concerning the possibility that an instantaneous value of a single wave signal will exceed the input range of the sampling means by sampling the sine wave signal by the sampling means and comparing the sampling values by means of the sampling means. The sampling means samples a sine wave signal which has a known frequency at N (3 or more) points per cycle. The comparison means is a means which compares the absolute values of the aforementioned samples with a threshold value which is set at sin (xcfx80/2xe2x88x92xcfx80/N) times one-half the input range of the sampling means when N is even, and sin [xcfx80/2xe2x88x92xcfx80/(2N)] times one-half the input range of the sampling means when N is odd. If the absolute value of at least one (1) sample is at or above the threshold, this apparatus infers the possibility that an instantaneous value of the aforementioned sine wave signal will exceed the input range of the sampling means.